Security considerations

This is a poor man's MAC. We use SHA256 to generate the second half of the UUID from the 64-bit
random looking input and the secret key. The small number of bits limits the security.

We will start getting collisions on the 64-bit random number after about 2^32 numbers are used.
But this just means we will be providing the function with the same input, so the same output
will be produced.

False Negatives

This is zero. If you produced the number with the mark function, this number will always be
detected with isMarked as long as you provide the correct key.

False positives

This is false detection. We worry about a UUID that was not generated using mark but is
detected as marked by isMarked. (A malicious adversary can always replay any UUIDs known as
marked. Thus, we will consider only new UUIDs.)

Assuming SHA256 is a perfect pseudo-random function, its truncated output, i.e. the last 64 bits of
the UUID, does not leak any information about the secret key. Given a fixed secret key, for any
64-bit input (corresponding to the the first half of the UUID), there is a unique 64-bit output
(corresponding to the second half of the UUID). There is only one such output per 64-bit input. So,
the probability of finding such input from a random draw is 2^(-64). The adversary would have more
than a 1/2 chance of finding it after 2^63 guesses.

Information leakage

The adversary can only know a UUID is marked if it is able to differentiate the output of truncated
SHA256 from a pseudo-random function. I am unaware of any significant results in doing so. The key
is 128-bits in length, so going through all possible values is currently unfeasible.